Optically diffuse structures often include a substrate such as polymer film or glass plate with a textured surface. These diffuse structures can be made reflective by coating a textured surface with an appropriate layer. Coating a diffuse substrate with a metal such as silver or aluminum, for example, will give a white diffuse reflector. Diffuse reflectors could be made colored by coating with an additional color filter layer that absorbs some of the visible spectrum. However, many applications, such as reflective displays, require that the wavelengths that are not reflected are instead transmitted. The use of a metal reflector layer prevents this.
An alternative reflective coating for a diffuse reflector is a multilayer dielectric mirror, which could be used in place of the metal coating and color filter. Dielectric mirrors are conventionally formed from alternating layers of materials with different refractive indices. In a standard design, the layer thicknesses are set to be a quarter of the wavelength of the desired peak reflection wavelength. Metal oxides are often used to achieve the required refractive indices, but for the most part, depositing metal oxides requires vacuum processes such as sputter coating. Each layer may need to be deposited separately and with high accuracy for the thicknesses and properties of the layer, and a typical dielectric mirror design requires at least 10 layers, often many more. The fabrication of a dielectric mirror can therefore be slow, expensive, and difficult to implement over large areas because of the need for processing in a vacuum. The resulting structures may also be brittle and thus are not ideally suited for applications that require robust or flexible parts.
Another alternative for a color mirror uses cholesteric polymers. Cholesteric polymers tend to self-organize into chiral periodic structure, and the period of the chiral structure can be matched to the optical wavelength to be reflected. A mirror fabricated using cholesteric polymers has the advantages that the polymers can be solution coated under atmospheric conditions, which may reduce fabrication costs, and the polymers generally form robust flexible films. However, it can be a challenge to make mirror structures involving cholesteric polymers diffuse. Cholesteric films are self-ordering. If the cholesteric polymer is simply coated onto a textured surface, the self-ordering might smooth out distortions create by the texture. As a result, mirror structures involving cholesteric polymer layers may not be as diffuse as required in some applications.
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